You can allow your visitors to leave comments in your gallery settings under Social > Allow Comments.
This is a separate content block that can be added to any page, gallery, or folder. You can find it in the Customize Design menu under Content > Social > Comments. Drag and drop the box onto the page where you want it to appear.
Content blocks can be positioned anywhere on your page. You can control the size and position as you wish.
This content block is NOT controlled by the gallery settings above.
Adding this content block to a page automatically turns comments on. You'll be able to choose the source gallery the comments are pulled from.
Who can comment?
To protect your site from spammers, visitors are required to log in to leave comments. They can use either a SmugMug account, Facebook, or Google to log in and leave a comment. A log-in is required and it's not possible to turn off that requirement.
Your commenters can also use the star-rating area to rate your photos if they choose. Once they've logged in using one of the above methods, they'll see five stars outlined below the comment box. They can click a single star or all five stars to show how much they love your photo!